1. Field of the Invention
The present invention concerns a novel antistatic composition comprising a semiconductor material, substantially amorphous vanadium pentoxide, V.sub.2 O.sub.5, in an aqueous solution. The composition is usable for the preparation of antistatic coatings and layers designed, in particular, for photographic, cinematographic, magnetic, and electrographic elements or products of manufacture, and for the preparation of antistatic fibers and filaments.
In the following, the invention is described in a general manner referring to photographic products. However, it is to be understood that the invention applies to all products for which the problem of removal of electrostatic charges arises.
2. Description of Prior Art
It is well known that numerous types of photographic film supports have the tendency to produce charges of static electricity during winding and unwinding, and that these charges do not easily dissipate, because the materials that are used as film supports usually are poor electrical conductors. High potentials that thusly have been created often discharge suddenly in the course of manufacture or in the course of utilization of the product by the user, causing flashes of light from static electricity and an undesirable recording of the static electricity discharge on a radiation-sensitive layer, such as a layer of photographic emulsion. In order to avoid this result, it is customary in the prior art to apply to the backing of the film support an electrically conductive layer, also referred to herein as an antistatic layer, to make it possible to facilitate the dissipation of the static charges, and thus avoid the sudden discharges and the resulting light flashes which otherwise would damage the radiation-sensitive layer.
Known antistatic layers generally consist of a binder in which is dispersed an organic or inorganic conductive substance to render the surface on which the layer is coated, for example, a film support, sufficiently conductive to make possible the flow of the electrostatic charges to a discharge means. Most often, antistatic layers are more or less hygroscopic layers, the efficiencies of which vary as a function of the degree of humidity in the air. Thus, such layers are not very suitable for use under conditions of low relative humidity because they are no longer sufficiently conductive. Likewise, such hygroscopic layers are not very suitable at conditions of high humidity because they become sticky. It is difficult then to separate them from the surfaces to which they adhere. The effort that is necessary to separate two superimposed layers, for example, sometimes creates charges higher than those that would appear in the absence of any antistatic layer.
In order to remove these electrostatic phenomena, the prior art suggests, e.g., the use of mixtures, such as those described in French Pat. No. 1,089,923, that comprise two compounds, one of these compounds being selected from among the alkyl phosphates of alkaline metals, and the salts of alkaline metals, of ammonium, or of amines, of cellulose sulfates or cellulose acetate-sulfate, the other compound being a spreading agent selected from among the higher fatty acid esters and polyalcohols. The prior art has also described use for this purpose of the alkaline or diester ammonium salts formed from ortho-phosphoric acids and aliphatic alcohols, such as those described in French Pat. No. 1,282,354.
Antistatic compositions also are known that comprise as an antistatic agent an alkaline metal salt having an atomic weight at least equal to 23, and an organic mono- or diester of phosphoric acid, in an organic or aqueous medium, and that may, in addition, contain a binder.
However, these known antistatic layers do not display all the qualities that are required in an antistatic layer, especially under changes in humidity conditions. They display, in fact, a surface receptiveness, e.g. coatability, that is often insufficient, a substantial variation in resistivity, as a function of the relative humidity, a loss of the antistatic effect after passage through processing baths in the case of photographic and cinematographic products, and a friction coefficient that is not easily adaptable to the type of utilization that is considered.
Trevoy, U.S. Pat. No. 3,245,833 issued Apr. 12, 1966, and U.S. Pat. No. 3,428,451 issued Feb. 18, 1969, describes electrically conductive supports comprising a semiconductor compound, e.g. cuprous iodide, or a complex of such compound, dispersed in a film-forming vehicle, and their use in radiation-sensitive elements. The cuprous iodide compounds are usually deposited from organic solvents. In contrast to such cuprous iodide systems, the V.sub.2 O.sub.5 of the present invention can be coated from an aqueous or hydro-organic system to form an antistatic layer using a water soluble binder.
In my copending patent application, Ser. No. 654,441 filed Feb. 2, 1976, now abandoned, I have disclosed antistatic compositions formed of a solution in water of a vitreous compound obtained by fusion of a glass-forming compound, such as an alkaline metal phosphate or polyphosphate, associated with one or more modifying compounds, which are transition metal oxides, such as V.sub.2 O.sub.5, which may display several states of valence in the final vitreous compound. Those vitreous compounds are obtained by casting the mixture of oxides in the melted state on a cooled metallic plate. The fused product is ground and dissolved to form a solution for use in an antistatic composition. The process of casting is generally satisfactory for preparing vitreous compounds having a low concentration of the modifying compound present. However, with a high concentration of modifying oxides present, which is of interest, since it is these modifying compounds that provide the desired conductive properties, the fused compositions become devitrified before solidification of the melted mixture. The resulting product in such cases is polycrystalline, heterogeneous, and usually insoluble.
Vanadium pentoxide, V.sub.2 O.sub.5, is a particularly interesting oxide for obtaining compositions having satisfactory conductive properties for the use in antistatic layers. One aim of the present invention is to avoid the above solidification problem and thereby to obtain an amorphous composition of vanadium pentoxide that one may easily apply in a thin layer to a surface of a support or substrate.
It is known that one may prepare colloidal vanadium pentoxide solutions and that such solutions are already the object of numerous studies. Thus, according to J. B. Donnet in the Journal de Chimie Physique (Journal of Physical Chemistry), No. 50, page 363 and following (1953), such solutions, which may be prepared by means of different processes (among which is the method of Erich Muller, consisting in projecting V.sub.2 O.sub.5 maintained at the melting temperature (about 700.degree.C.) in cold distilled water), contain particles the chemical nature of which has caused a certain controversy. Donnet teaches that the dried product of colloidal solutions of V.sub.2 O.sub.5 is crystalline (page 364). Erich Muller, in an article published in Kolloid Z. 8 P. 302 (1911), mentions that V.sub.2 O.sub.5 dissolves very little in water and that it is possible to obtain a colloidal solution by projecting, as indicated, previously, V.sub.2 O.sub.5 maintained at the melting temperature into cold distilled water. However, the dried product obtained from Muller's process cannot be redissolved in distilled water.
A. Revcolevschi, during a conference held Dec. 13, 1973, before the Society for the Encouragement of National Industry, (France) described a then novel method for obtaining amorphous structures of oxides or of mixtures of oxides. J. Material Sciences, Vol. 8, P. 1359 (1973). His process, called hyperquenching, consists in rapidly cooling a metal oxide in the liquid or vapor state by contacting it with a heat conductive surface in a method analogous to "splat-cooling." "Splat-cooling" was first developed by P. Duwez et al, J. Appl. Phys. V. 31, P. 1136, (1960) for quenching metals and alloys. For a hardening from the liquid state to be efficient, it is necessary that the speed of cooling of the material be exceptionally high at the moment of transition from liquid to solid. This hyperquenching step requires an extremely high speed of heat-exchange at this precise moment. Duwez et al have shown that the mechanism of rapid heat-exchange by means of conduction is the most efficient method, if the following conditions are respected:
the substratum on which the hardening is carried out is an excellent heat-conductor;
the heat contact is as perfect as possible;
the distance between the liquid and the substratum is as small as possible;
the time for the passage from the liquid state to the solid state is as short as possible.
Revcolevschi, ibid, proposes different processes to obtain the above result. It was possible to obtain, with the described processes, amorphous metal oxide structures, because the speed of hardening attained was sufficiently rapid so as to solidify the liquid-state structure. Also, Revcolevschi teaches that it is possible to obtain amorphous phases from oxides or mixtures of oxides, such as V.sub.2 O.sub.5, TeO.sub.2, MoO.sub.3 and WO.sub.3. However, the methods of rapid cooling described by Duwez et al and by Revcolevschi are taught to be used to form solid glasses by a rapid liquid-to-solid state transition. No formation of a colloidal dispersion of an oxide or of a mixture of oxides is taught or suggested by the authors. Although Duwez et al, ibid, state that the classical method for achieving high rates of cooling consists of injecting a small droplet of molten alloy into a liquid quenching bath, neither Duwez et al or Revcolevschi teach or suggest formation of a colloidal dispersion of V.sub.2 O.sub.5 by a "splat-cooling," or hyperhardening method or rapid casting and quenching of fused metal oxides into water to thereby achieve formation of an amorphous V.sub.2 O.sub.5 which also is in colloidal solution form.
In the vanadium glass making art, Patterson et al, U.S. Pat. No. 3,839,231 issued Oct. 1, 1974, describe a vanadium glass frit made by melting or fusing together in air vanadium pentoxide and normal glass constituents well known in the glass making art, e.g. other metal oxides besides V.sub.2 O.sub.5, and pouring the melt into water to form a frit or grinding the fused mass to recover a millable powder. The frit or powder then is milled to a desired fineness and used to prepare pastes in liquid vehicles for use in making fired electrical elements having semi-conductive to metallic behavior.